In an electrical double layer capacitor having an electrolytic solution between a pair of electrodes, charging, and discharging are physically conducted by adsorption and desorption of ions, which differs from a secondary battery in which charging and discharging are conducted by chemical reaction. Therefore, in discharging, electric current can be quickly taken out, and charging can be completed rapidly.
An electrolytic solution to be used for an electrical double layer capacitor are required to have various characteristics, and one of them is high withstanding voltage, namely, an electrolytic solution is required to have resistance to degradation (reduction decomposition) when exposed to high voltage for a long period of time.
For improving withstanding voltage of an electrical double layer capacitor, JP64-14882A proposes secondary power source which has an upper limit voltage of 3V and is equipped with a positive electrode comprising activated carbon as main component and a negative electrode made of carbon material having 0.338 to −0.356 nm of a face-to-face distance of [002] faces by X-ray diffraction and previously occulted with lithium ion. In addition, JP8-107048A and JP9-55342A propose secondary power source using, on a negative electrode, carbon material being capable of undergoing adsorption and desorption of lithium ion and previously subjected to occlusion with lithium ion by a chemical method or an electrochemical method. However, in the case of using carbon material undergoing occlusion of lithium ion on a negative electrode, it is necessary to previously conduct doping of lithium ion, which leads to high cost, and in addition, large swelling and shrinkage of the carbon material occur and cycle characteristics are not good.
Example of another method of improving withstanding voltage of an electrical double layer capacitor is the use of quaternary onium salt having polyfluoroalkyl group as an electrolyte as disclosed in JP2002-222739A. However, in the case where a fluorine content of this quaternary onium salt is small, an effect of improving withstanding voltage is low, and, for example, in the case of use for an electrical double layer capacitor, it only has oxidation resistance up to 2.8V, and thus, it cannot be said that withstanding voltage is improved. Further, solubility is low and ionic conductivity is also low.
Further, JP2002-260966A discloses an electrical double layer capacitor using, as an electrolyte, a salt prepared by introducing fluorine atom to an imidazolium salt. However, even in this electrical double layer capacitor, a significant effect of improving withstanding voltage cannot be seen by the introduction of fluorine atom, and the electrical double layer capacitor has only withstanding voltage up to 3.3V.
As mentioned above, there are trials of improving withstanding voltage of an electrical double layer capacitor such as the use of carbon material undergoing occlusion of lithium ion as a negative electrode and modification of an electrolyte salt, but these trials are not always sufficient.
On the other hand, in the case of an electrical double layer capacitor using polarizable electrodes on both of a positive electrode and a negative electrode, there are trials of improving withstanding voltage of an electrical double layer capacitor by improving withstanding voltage of a solvent by using a fluorine-containing carbonate (for example, refer to JP8-222485A, JP9-148197A, JP9-7896A, JP9-251861A and JP10-233345A). However, in these techniques, while there is improvement in withstanding voltage of an electrical double layer capacitor by the use of a fluorine-containing carbonate, even in the case of using polarizable electrodes on both electrodes, withstanding voltage is up to 3.3V at most, and further improvement in withstanding voltage up to 3.5V and 4.0V cannot be desired.